Te British prime minister has threatened that his country will exit the European Union if former Luxembourg premier Jean-Claude Juncker becomes the president of the European Commission.

According to German news magazine, Der Spiegel, David Cameron made the comments on the sidelines of an EU summit in Brussels on Tuesday.

He reportedly told German Chancellor Angela Merkel, who supports Juncker for the post, that the appointment would “destabilize his government to such a point that it would bring forward a referendum on whether to exit the EU.”

Political analysts say a referendum on EU membership in the UK, which is slated to be held in 2017, will probably lead to a no-vote.

Cameron, who believes that a reformer must take the EU’s top post, reportedly said, “A figure from the 80s cannot resolve the problems of the next five years.”

Some other EU leaders, including Swedish Premier Fredrik Reinfeldt and Hungarian Premier Viktor Orban, have also voiced their opposition to Juncker.

Merkel, however, threw her weight behind Juncker on Friday.

The EPP “with its top candidate Jean-Claude Juncker has become the strongest political power which is why I am now conducting all talks exactly in this spirit, that Jean-Claude Juncker should become president of the European Commission,” Merkel said.

The European People’s Party (EPP), to which both Juncker’s and Merkel’s parties belong, won 213 out of 751 seats in the European Parliament elections last week.

The group chose Juncker as its candidate for the presidency succeeding Jose Manuel Barroso.

EU leaders traditionally choose the commission’s head on their own, but under current rules, they have to take into account the results of the European Parliament elections.